The robot has been silent for over a year now, but made revolutionary contributions to space science in its time

After six
years of active exploration and one year of silence on Mars, NASA has
finally decided to call it quits for one tough robot that has been put to rest.

Spirit, a golf cart-sized, solar-powered robot geologist that was sent to Mars
in 2004, spent six long years traveling the Martian surface. But after enduring
many harsh winters on Mars, Spirit finally fell silent last year, and will be
removed from the mission next week.

In January 2004, Spirit and its twin, Opportunity, were parachuted onto
opposite sides of Mars for what was supposed to be a three-month expedition.
The robots were designed to scale craters and hills while drilling into rocks
along the way. The robots would then record data to their flash memory, which
would be sent to deep space antennas in California, Australia and Spain.

Spirit and Opportunity were very popular back on Earth. People would follow
their journey through every crater and rock, eager to hear what was found on
the Red Planet.

But the robots were more than just machines feeding NASA scientific feedback.
They seemed to take on different personalities, with Opportunity being the
overachiever and Spirit being the troublesome and sometimes misguided sibling.
When the robots first landed on Mars, Opportunity ended up in an ancient
lakebed that was rich with water-forming materials, showing that Mars, which is
now a dry planet, was once wet and tropical billions of years ago. Spirit, on
the other hand, landed in a crater named Gusev that was the size of Connecticut
and possessed very little evidence of a watery past for Mars.

Not long after landing, Spirit began sending "nonsense data" back to
Earth and fell into critical condition, but NASA engineers were able to bring
Spirit back to health.

But in 2006, Spirit showed signs of complications once
again. One of its front wheels stopped spinning, which made Spirit travel backward
and drag the bum wheel. It also started to "forget" to save data to
its flash memory. Then, in April 2009, Spirit got stuck in a sand pit, and
engineers were unable to get it out. Spirit was forced to remain stuck there
until the present time, but in the early days of its self-inflicted solitary
confinement, Spirit was still able to send data back about rocks within reach.

Engineers blame the harsh Martian winters for Spirit's continuous problems. At
times, Spirit's internal temperature was minus 67 degrees Fahrenheit, and
engineers had no way of tilting the robot toward the low winter sun, which
would have warmed Spirit through its solar panels.

In January 2010, NASA officially announced the end of Spirit's career. It was a
stationary spacecraft at that point, and began to fall silent.
For over a year, orbiting spacecraft attempted to provoke any sign of life from
Spirit, but there was no such luck.

NASA plans to make one last effort to make contact with the robot Wednesday,
and a final farewell is planned next week after Memorial Day. Engineers expect
the farewell to be "more like an Irish wake than a funeral" in
celebration of Spirit's long life and revolutionary contributions to space
science.

Over its six years of activity, Spirit traveled 4.8 miles.

"This is a story of perseverance," said Jim Bell, a mission scientist
at Arizona State University. "Spirit got a lot of bum breaks, including
almost dying early on and wheel problems."

While Spirit's journey is over, the exploration of Mars is not. Opportunity is
still scaling Mars' surface at full health, and has traveled 18.5 miles since
landing in 2004. It is now journeying toward a crater called Endeavour, which
is less than three miles away from its present location. NASA expects the robot
to make it to the crater's rim by the end of the year.

In addition, NASA plans to send a Mini Cooper-sized mega-rover onto Mars in
Summer 2012 to accompany Opportunity in its exploration.

The new RTG powered Mars Science Laboratory doesn't drive all night either, it stops to recharge it's batteries at night. The power demands of the motors exceed the power available from the RTG so the MSL uses batteries much like Spirit and Opportunity do.

The big advantage is reliable power and heat. Power is not dependent on dust storms, dust accumulation, or sun angle like the MERs were. It can operate at full capacity in shadow and winter time. The MERs were sometimes stuck for 5 months at at time during the Martian winter waiting for solar power to be strong enough to move the rover. It was crucial to retain enough battery power to keep the internal systems from freezing at night. On the MSL waste heat from the RTG is used to keep the systems warm. Dust accumulation on the solar panels also cut solar power considerably. Dust storms were another big threat not just from the threat of dust accumulation but the airborne dust blocked out most of the sunlight for weeks at a time.http://www.nasa.gov/images/content/182691main_mer-...

The major drawback of RTGs is cost. They are very expensive, not only in materials cost, but assembly, testing, and safety procedures and safety qualification of everything down the line, especially the launch vehicle.

Within the budget window for Spirit and Opportunity and the original plan for only a 90 day mission, solar was a much cheaper (and lighter weight) solution. The much more expensive Flagship class MSL is designed for a 2 year mission.

The other major problem with RTGs is the very limited supply of Plutonium-238. After the launch of the MSL later this year the total world stockpile will be less than 20Kg, and there is no longer any facility that can produce it. As a reference point MSL uses about 4kg, New Horizons (the Pluto probe) uses 7.8 kg, the Cassini probe (Saturn)uses 32.7 kg.

Any probe to Jupiter or beyond must use RTG power, as sunlight is too faint at that distance to use solar.

RTG or no RTG all comes down to money, people with anti-nuclear sentiments actually have little to do with it. RTG's are designed to be extremely robust. A launch failure dropped one in the ocean off the town I grew up in, they recovered it from the bottom of the ocean, refurbished it and flew it again.